Method for the purification of hydrogen peroxide solutions

ABSTRACT

The invention relates to a novel chromatographic process for the further purification of hydrogen peroxide solutions, giving high-purity solutions which can be employed in semiconductor technology under today&#39;s high purity requirements.

The invention relates to a novel process for the further purification ofhydrogen peroxide solutions which gives high-purity solutions that canbe employed in semiconductor technology under today's high purityrequirements.

In the production of highly integrated electric circuits, extremely highpurity requirements are placed on the chemicals used. While theproduction of 1 megabit chips tolerates a chemical quality withimpurities in the low ppm region, the production of 4 to 16 megabitchips requires chemical qualities with a maximum impurity level in therange below 10 ppb.

One of the key chemicals in chip production which has to meet thesepurity requirements is hydrogen peroxide. Since the latter is preparedvirtually exclusively by the anthraquinone process and is usuallypurified and concentrated by rectification in aluminium orstainless-steel columns, it does not have the requisite purity. Due tocontact with the plant parts, the distillate is contaminated, inparticular, with aluminium or also other metals. In addition, itcomprises residues of organic carbon compounds (“organic C”), such assolvents (alcohols, ketones, aliphatic hydrocarbons and acids), and ofanthraquinone derivatives as a consequence of the process. For use inmicroelectronics, the hydrogen peroxide must therefore be subjected toeffective post-treatment in order to reduce the cation, anion and carboncontent to the requisite purity level.

Purification of hydrogen peroxide solutions by distillation alone doesnot achieve the requisite purity with respect to metallic impurities andcarbon. For example, the solutions comprise readily volatile orsteam-volatile organic carbon compounds from the anthraquinone process,which cannot be separated off in a simple manner by distillation. Thecontent of dissolved organic carbon in the hydrogen peroxide can havevalues of up to 150 mg/l. However, metal ions and carbon impurities inthe hydrogen peroxide have a particularly interfering effect in theproduction of microchips, these impurities having an all the morecritical effect the more highly integrated the chips to be produced.There has therefore been no lack of attempts in the prior art to removethe impurities from the hydrogen peroxide by post-treatment with cationand/or anion exchangers.

Ion-exchanging materials which have been proposed for this purpose arering-substituted aromatic hydrocarbon cation exchanger resins for theremoval of cations and aromatic hydrocarbon anion exchanger resinscontaining tertiary amino or ammonium groups or pyridine rings for theremoval of anions. The functional groups present in these ion exchangerresins often make the ion exchanger resins so sensitive to oxidationthat the purification of hydrogen peroxide using these ion exchangerresins has to be carried out at relatively low temperatures of about 0°C. and with particular precautionary measures.

In order to circumvent the problem of reaction with oxidation-sensitivegroups, U.S. Pat. No. 5,268,160 proposes further purification usingnonionic organic hydrophobic adsorber resins based on crosslinkedpolystyrene resins. However, this only gives hydrogen peroxide solutionswhich comprise a multiple of the tolerated impurities and are thereforenot suitable for use in chip production in accordance with today'sstandards.

The high oxidation sensitivity of ion exchanger resins is attributableto the fact that, in the presence of heavy metals, such as, for example,Fe or Cu, etc., hydrogen peroxide is able to form hazardous hydroxylfree radicals, which oxidatively attack the carbon skeleton of the ionexchanger and are able to form readily decomposable epoxides orhydroperoxides therewith. The epoxides or hydroperoxides formed candecompose not only explosively, but under certain circumstances even inthe manner of a detonation. The use of cation exchangers or anionexchangers for the purification of hydrogen peroxide solutions is thusproblematic and requires particular care.

In order to circumvent this problem, EP-A1-0 502 466 and DE-A1-38 22 348A1 have described processes for the further purification of hydrogenperoxide solutions in which metal ions present after purification bydistillation are separated off from corresponding solutions by means ofchelating agents and by means of non-ion-exchanging polymeric adsorbentsbased on styrene-divinylbenzene copolymers. However, this process isafflicted with the disadvantage that undesired chemicals must again beadded to a pre-purified solution and subsequently have to be separatedoff again.

DE-A1-42 14 075 discloses a process in which the hydrogen peroxidesolutions to be purified are treated with an anion exchanger and anonionic adsorber resin in order to separate off organic impurities. Inthis process, the hydrogen peroxide solutions to be purified are treatedwith a cationic resin in the acidic form and subsequently with amedium-strength anionic resin in the basic form at 0° C. This isfollowed by treatment with an adsorber resin having a microreticularstructure, i.e. with a nonionic resin. It has been found that hydrogenperoxide solutions treated in this way no longer satisfy today'srequirements of the semiconductor industry, since the concentration ofthe organic impurities still present in the solutions is too high.

U.S. Pat. No. 4,879,048 again discloses a process for the furtherpurification of hydrogen peroxide solutions by reverse osmosis. However,the life of the semipermeable membrane causes problems. In addition,today's purity requirements are not met.

The object was therefore to provide a process for the furtherpurification of hydrogen peroxide solutions which is simple to carry outand which enables the concentration of organic impurities (TOC) to bereduced to less than 5 ppm and at the same time allows interfering metalions to be separated off.

The object according to the invention is achieved by a process for thefurther purification of hydrogen peroxide solutions by treating thehydrogen peroxide solutions to be purified, which have concentrations inthe range 5-59%,

a) with an anion exchanger resin,

b) with a nonionic adsorber resin in the form of a hydrophobic aromatic,crosslinked polymer having a macroporous structure, and

c) with a neutral adsorber resin from the group consisting of thestyrene-divinylbenzene resins having a highly macroporous structure, thelatter having been formed by pyrolysis treatment of the resin,

with the proviso that the treatment with the adsorber or exchangerresins can be carried out in any desired sequence, but with thecondition that the treatment with the neutral adsorber resin takes placein the final step.

The anion exchanger resin selected in accordance with the invention canbe a resin from the group consisting of strongly or weakly basicstyrene-divinylbenzene resins containing quaternary ammonium groups asfunctional groups and strongly or weakly basic styrene-divinylbenzeneresins containing tertiary amino groups as functional groups.

The nonionic adsorber resin used in accordance with the invention is anaromatic crosslinked polymer having a macroporous structure, inparticular selected from the group consisting of styrene-divinylbenzeneresins having a macroporous structure and a large surface area.

In an additional purification step of the process according to theinvention, the neutral adsorber resin used is a resin selected from thegroup consisting of styrene-divinylbenzene resins having a highlymacroporous structure and a moderate surface area.

In order to carry out the process, the hydrogen peroxide solution to betreated is passed, in accordance with the invention, throughchromatography columns connected in series at a flow density of from 0.2l/h cm² to 1.0 l/h cm², in particular from 0.5 to 0.7 l/h cm²,.

If the further purification is carried out in fluidized beds connectedin series, a residence time in the range from 0.008 to 20.0 min isadvantageous.

The further purification of these hydrogen peroxide solutions is carriedout at temperatures of from 15 to 25° C., preferably at 20° C.

The process is particularly advantageous and economical under continuousconditions. However, it can also be carried out in batch operation.

The object can also be achieved in accordance with the invention by acorresponding process in which the hydrogen peroxide solution to bepurified is passed into fluidized beds connected in series whichcomprise, separately from one another,

a) an anion exchanger resin,

b) a nonionic adsorber resin, and

c) a neutral adsorber resin,

with a residence time of from 0.0008 to 20.0 min, where the hydrogenperoxide solution to be further purified is in each case separated offfrom the exchanger or adsorber resins by filtration.

The further purification in fluidized beds connected in series can, inaccordance with the invention, be carried out at temperatures of from 0to 20° C., in particular at from 0 to 10° C., and can be carried outeither in batch operation or continuously. As in the case of furtherpurification in columns, from 5 to 59% hydrogen peroxide solutions canbe employed in the process according to the invention.

The hydrogen peroxide solutions employed in the process according to theinvention are solutions which have been pre-purified by distillation andcomprise, as impurities, only very small amounts of ionic inorganicimpurities, such as, for example, metal cations Al, Fe, Zn, etc., oranions, such as NO₃ ⁻, PO₄ ²⁻, etc., and organic impurities as aconsequence of the preparation.

Experiments have shown that up to 95% of the undesired organicimpurities present can be removed without difficulties fromcorresponding 5 to 59% hydrogen peroxide solutions by successivechromatographic treatment with anion exchanger resins, nonionic adsorberresins and neutral adsorber resins. For example, the TOC content of a50% hydrogen peroxide solution can be reduced from 40 ppm to less than 5ppm by the process according to the invention, so that the resultantsolution has a purity which is absolutely necessary for use in thesemiconductor industry under current requirements.

It has been found that the reduction in the TOC content only takes placedue to the treatment with the three different resins mentioned above inthe desired manner. A reduction in the content in the solutions merelyby treatment with the neutral adsorber resins according to the inventiondoes not result in the requisite purification in a process to be carriedout on an industrial scale, since the adsorption capacity of theseresins for organic constituents is limited and it would not be possibleto carry out a corresponding purification in an economical manner. Bycontrast, a combination consisting of purification by means of apreferably strongly basic anion exchanger resin, a nonionic adsorberresin and a specific neutral adsorber resin gives an excellentpurification result.

Hydrogen peroxide solutions, which may have been pre-purified bydistillation, can be further purified either by successive contact withthe exchanger resin and the various adsorber resins by mixing inseparate fluidized beds, but preferably by contact with thecorresponding resins in packed columns. The flow rate of the hydrogenperoxide solutions should be set so that the content of carbon and ofthe ionogenic impurities in the eluate does not exceed the maximumamount that can be tolerated. Flow densities of from 0.2 to 1.0 l/h cm²,in particular from 0.5 to 0.7 l/h cm², are advantageously set.

The purified hydrogen peroxide flowing out of the adsorption column iscollected in a suitable container. If the further purification iscarried out in suitable fluidized beds, the hydrogen peroxide solutionis separated off by filtration and collected in a suitable container.However, the residence time here should be set so that althoughadsorption of undesired impurities takes place, reactions with theresins do not. It has been found that under suitable conditions, i.e. ata temperature of from 0 to 20° C., preferably between 0 and 5° C., atatmospheric pressure and a residence time of the hydrogen peroxidesolutions of between 0.008 and 20.0 min, good further purificationresults are obtained, but at the same time no reaction with theexchanger resins is observed on the basis of changes in the oxygencontent of the hydrogen peroxide solutions or on the basis of warming.

The successive treatment with different resins can be carried out in anydesired sequence. Particularly good results are achieved owing to theadsorption capacities if the neutral adsorber resin is employed in thefinal purification step. However, very particularly good results areachieved if the sequence anion exchanger resin, nonionic adsorber resinand subsequently neutral adsorber resin is observed. This sequence isparticularly important since the adsorption capacity of the neutraladsorbent would be the limiting factor in the process and complexoptimization of the volume flow rates in the various purification stepsand the ratio of the column volumes to one another would be necessary.However, if the columns are connected one after the other in thepreferred manner, this work is superfluous. In particular if thetreatment with the neutral adsorber resin is carried out last, thisprocess parameter is unproblematic.

Strongly basic anion exchanger resins which can be employed are thosebased on styrene-divinylbenzene. For example, a corresponding resin iscommercially available under the trade name Amberlyst A-26®(manufacturer Rohm & Haas). The active groups in this resin are—N(CH₃)₂.Cl. Further resins containing the same active groups areAmberlyst A-15®, Amberlyst A-21® and Amberlyst A-27®. Other suitableresins are Amberjet® 4200 Cl, Amberjet® 4400 Cl, Amberlite® IRA 402 Cl,Amberlite® IRA 404 Cl, Amberlite® IRA 900 Cl, Amberlite® IRA 904 Cl,Amberlite® IRA 400 Cl, Amberlite® IRA 410 Cl, Amberlite® IRA 420 Cl,Amberlite® IRA 440 Cl, Amberlite® IRA 458 and Amberlite® 16766. Likewisesuitable are the weakly basic anion exchanger resins IRA-35, IRA-93,IRA-94 and IRA-68 sold under the trade name Amberlite®. It is alsopossible to employ the anion exchanger resins commercially availableunder the names Dowex, Diaion Type I and Type II, and Duolite, which maybe either strongly or weakly basic.

Although, according to prevailing opinion, the functional groups of thesaid anion exchangers are oxidatively attacked by hydrogen peroxidesolutions, experiments have shown that this can be completely orvirtually completely prevented by setting suitable operating parameters.Depending on the solution to be treated, this can take place by settinga high volume flow rate and/or by corresponding cooling. If necessary,the process is carried out with cooling to about 0° C. However, it hasbeen found that this is usually only necessary if solutions with arelatively high content have to be further purified. In the case offurther purification of solutions in the lower concentration range, thisis superfluous since firstly reactions can be kept at a relatively lowlevel or prevented by setting a suitable volume flow rate, and secondlylocal temperature changes can be suppressed.

Nonionic adsorber resins which can be employed in the process accordingto the invention are those based on styrene-divinylbenzene having amacroporous structure and a large aromatic surface area. Correspondingresins are free from constituents that can be washed out, such as, forexample, monomers or polymerization aids. These adsorbents have no ionicfunctional groups and are thus completely nonionic hydrophobic polymerswhose adsorptive properties are based exclusively on the macroporousstructure, the wide range of pore sizes, the unusually large surfacearea and the aromatic nature of this surface. These adsorbents are thusclearly distinguished from cation and anion exchangers, which, owing totheir functional groups present on the surface, are very sensitive tooxidation. Nonionic adsorber resins adsorb and liberate ionic speciesthrough hydrophobic and polar interactions, i.e. they have high affinityto hydrophobic organic substances, but only low affinity to hydrophilicsubstances, such as water or hydrogen peroxide.

Corresponding resins are marketed commercially, for example, under thenames Amberlite XAD-4®, a hydrophobic polyaromatic resin, AmberliteXAD-2® and Amberlite XAD-16®, likewise a hydrophobic polyaromatic resin,the moderately polar acrylic resins Diaion HP2MG and Diaion HP2MG andDiaion HP22SS®, a more finely divided version of specification HP20.These adsorber resins have continuous polymer phases and particularlyregular pores. They are stable in pH ranges from 0 to 14 and totemperatures of up to 250° C. Under process conditions, these resins areactive both at ambient temperatures, i.e. at temperatures of from 20 to30° C., and at lower temperatures, such as, for example, 0° C. or lower.

The successive treatment with a basic anion exchanger resin and anonionic adsorber resin enables virtually complete removal of polar andoptionally ionic impurities from hydrogen peroxide solutions whileprotecting the resins employed to the greatest possible extent.

Suitable neutral adsorber resins are, for example, those based oncarbonized styrene-divinylbenzene resins having a highly macroporousstructure and a moderate surface area. Such resins are, for example,commercially available under the trade name Ambersorb®. Ambersorb® 563,Ambersorb® 564, Ambersorb® 572, Ambersorb® 575, Ambersorb® 600 andAmbersorb® 1500 can be employed in the process according to theinvention. These different specifications are carbonized adsorbentsprepared from highly sulfonated, macroporous styrene-divinylbenzene ionexchanger resin which has been pyrolized in a special process. As aconsequence of their preparation process, corresponding adsorbers haveuniform porosity, constant hydrophobic properties and excellentmechanical stabilities.

Experiments have shown that only the combination of the purificationsteps with the described treatment with neutral adsorber resins in thefinal step with in each case one treatment with anion exchanger resinsand one with nonionic adsorber resins is suitable for reducing thecontent of organic impurities (TOC) in hydrogen peroxide solutions tolevels which satisfy the high quality requirements of the semiconductorindustry, i.e. to TOC values of <5 ppm, better <1 ppm.

In this connection, it has also been found that it is precisely theparticular properties of the neutral adsorber resins that areresponsible for the reduction in the content of organic impurities.

The difference between nonionic and neutral adsorber resins will be madeclear in an illustrative manner in the following table using the exampleof the nonionic adsorber resin Amberlite XAD-4 and the neutral adsorberresin Ambersorb 563:

TABLE 1 Nonionic adsorber Spec. neutral adsorber Name resin resinExample Amberlite XAD 4 Ambersorb 563 Matrix Styrene-DVB Styrene-DVB;post- treated by pyrolysis Surface area m²/g 750 550 Porosity g/ml 0.50.6 Micropore/macropore >1 1 ratioSpecial neutral adsorber resins which can be employed in the processaccording to the invention thus have the following product propertieswhich distinguish them from conventional nonionic adsorber resins asfollows:

-   -   high macroporosity, where the micropore:macropore ratio can        adopt a value of up to 1, and the porosity is >0.55 g/ml at a        surface area/weight unit ratio of less than 600 m²/g    -   excellent mechanical stability and chemical resistance    -   due to the relatively high proportion of macropores, the        adsorber resin is more accessible (more effective) to relatively        high-molecular-weight organic components.

Before use of the exchanger and adsorber resins in the process accordingto the invention, it is advisable to free the resins from impurities asa consequence of the preparation by means of suitable, pure solventsknown to the person skilled in the art for this purpose, sinceimpurities of this type could in some cases decompose hydrogen peroxide.For pre-washing nonionic adsorber resins, it is possible to use, forexample, lower alcohols, preferably methanol. Anion exchanger resinsthat can be employed in accordance with the invention can, for example,be pre-washed with 2-propanol and subsequently ultrapure water, whileneutral adsorber resins can be pre-washed with steam and subsequentlyultrapure water.

The process according to the invention can be carried out in batchoperation, in which case the exchanger and adsorber resins used areregenerated each time a certain amount of hydrogen peroxide solution hasbeen further purified. However, it is also possible to carry out theprocess continuously, for example by columns of the same charging beingpresent in parallel to the columns currently being used and which can beswitched to by redirecting the volume flow on saturation with theimpurities to be removed.

In this way, each column can be regenerated individually, the volumeflow does not have to be interrupted, and no wasted running times occur.The limiting factor is no longer the adsorption capacity of the resinsemployed.

The combinations of anion exchanger resin and adsorber resins givenbelow in Table 1 are highly suitable for carrying out the processaccording to the invention. The combinations shown are given by way ofexample and should not be regarded as limiting for the presentinvention.

TABLE 2 Step 1: Step 2: Step 3: Anion exchanger Nonionic ad- Neutraladsorber Process resin sorber resin resin Rohm & Haas Amberlyst ®Amberlite ® Ambersorb ® 563 product name A-26 XAD-2 Amberlyst ®Amberlite ® Ambersorb ® 564 A-27 XAD-4 Amberjet ® Amberlite ®Ambersorb ® 572 4200Cl XAD-16 Amberjet ® Ambersorb ® 575 4400ClAmberlite ® Ambersorb ® 600 IRA402Cl Amberlite ® Ambersorb ® 1500IRA404ClThe process according to the invention is carried out under conditionsand using methods which are known per se to the person skilled in theart. Good purification results are achieved using columns which have aratio between column height and column diameter of between 7.5:1 and2.5:1, preferably between 6:1 and 4:1, particularly preferably 5:1, andthrough which from 3 to 5 times the bed volume of hydrogen peroxidesolution flows per hour. However, the process can also be carried out incolumns which have heights of from 10 to 200 cm and diameters of from 1to 2 cm. For purification of relatively large amounts, however, columnshaving heights of from 2.5 to 4 m and diameters of from 0.50 to 0.8 mare particularly suitable.

An important factor for the success of the purification processaccording to the invention is that all equipment and containers employedduring the purification consist of suitable materials so that the highlypure hydrogen peroxide is not subsequently re-contaminated by, forexample, metal ions, etc., from the containers and piping. Suitablematerials have proven to be, in particular, borosilicate glass,polytetrafluoroethylene, polyvinylidene fluoride and high-pressurepolyethylene.

The present invention provides a particularly simple and advantageousprocess for the purification of hydrogen peroxide for applications inmicro-electronics. The process according to the invention isdistinguished, in particular, by the fact that even very low contents oforganic impurities in hydrogen peroxide solutions can be reduced veryeffectively and in addition particularly interfering cations, such asNa, K, Mg, Al, Ca, Fe, Zn and Cu, are also removed virtually completely.

The hydrogen peroxide solutions further purified by the processaccording to the invention have increased stabilities and conform totoday's purity requirements for the production of highly integratedchips.

By means of the description of the invention given here, the personskilled in the art will readily find it possible to prepare highly purehydrogen peroxide solutions which meet the high requirements for use intoday's chip production methods. The examples given below are intendedto serve for better understanding of the present invention, but are notsuitable for restricting the invention thereto.

Working Examples, Methods and Results:

In order to demonstrate the efficiency of the process according to theinvention, the following aqueous hydrogen peroxide solutions andexchanger and adsorption resins were employed and the followinganalytical methods were used in the following examples:

Hydrogen Peroxide Solutions:

-   Origin: Merck KGaA, anthraquinone process (“autoxidation process”)    Batch KD09971042-   Concentration: 50%±1% or 30%±1%-   Amount: 2.5 l    Adsorber Resins:

Anion exchanger resin: Amberlyst ® A 26 Nonionic adsorber resin:Amberlite ® XAD-4 Neutral adsorber resin: Ambersorb ® 563 Pretreatment:All resins were rinsed with ultrapure water for 8 hours before useAnalytical Methods:

TOC determination using Shimadzu TOC 5000 (measurement method based oncomplete decomposition of the sample on a platinum catalyst at elevatedtemperature). The carbon dioxide formed therefrom is determined in totalby means of an infrared spectrometer. Cations and anions were notdetermined specifically.

EXAMPLE 1

-   -   Flow rate: 1.0 l/h    -   Flow density: 0.3 l/h cm²

TABLE 3 Stage: adsorber TOG [ppm] Before 1st stage (reference H₂O₂ 50%)38.0 1st stage: Amberlyst ® A 26 23.7 2nd stage: Amberlite ® XAD-4 473rd stage: Ambersorb ® 563 2.4

The experiment confirms the excellent mechanical stability and chemicalresistance of the 3rd stage, even to relatively highly concentratedhydrogen peroxide solutions, and shows that the 3rd stage achieves afurther significant purification effect in the TOC region <5 ppm.

EXAMPLE 2

-   -   Flow rate: 1.0 l/h    -   Flow density: 0.3 l/h cm²

TABLE 4 Stage: adsorber TOC [ppm] After 1st stage (reference H₂O₂ 30%)11.5 Only Amberlite ® XAD-4 1.6 Only Ambersorb ® 563 1.1 FirstAmberlite ® XAD-4, then Ambersorb ® 563 0.4This experiment shows that the nonionic adsorber resin and the specialneutral adsorber resin have different selectivities in the reduction inthe concentration of relatively high-molecular-weight organiccomponents. Only when the two adsorber resins are used in combination isthe content of organic components in hydrogen peroxide solutions (TOC)reduced to less than 1 ppm.

1. A process for purifying a hydrogen peroxide solution having aconcentration of 5-59%, comprising treating the solution a) with ananion exchanger resin, b) with a nonionic adsorber resin of ahydrophobic aromatic, crosslinked polymer having a macroporousstructure, and c) with a neutral adsorber resin of astyrene-divinylbenzene resin having a macroporous structure, made by apyrolysis treatment of the resin, with the proviso that the treatmentwith the adsorber or exchanger resin can be carried out in any desiredsequence except the treatment with the neutral adsorber resin takesplace in the final step.
 2. A process according to claim 1, wherein theanionic exchanger resin is a weakly or strongly basicstyrene-divinylbenzene resin containing quaternary ammonium groups asfunctional groups or a weakly or strongly basic styrene-divinylbenzenecontaining tertiary amino groups as functional groups.
 3. A processaccording to claim 1, wherein the nonionic adsorber resin is astyrene-divinylbenzene resin having a macroporous structure.
 4. Aprocess according to claim 1, further comprising passing the hydrogenperoxide solution through columns for performing treatment a), b), andc) connected in series at flow density of 0.2-1.0 l/h cm².
 5. A processaccording to claim 1, further comprising passing the hydrogen peroxidesolution through fluidized beds for performing treatment a), b), and c)connected in series with a residence time of from 0.008-20.0 min.
 6. Aprocess according to claim 1, wherein the process is carried out at15-25° C.
 7. A process according to claim 1, wherein the process iscarried out continuously.
 8. A process according to claim 1, wherein theprocess is carried out in a batch operation.
 9. A process according toclaim 1 wherein the process is carried out at 0-20° C.
 10. A processaccording to claim 1, further comprising passing the hydrogen peroxidethrough columns for performing treatment a), b), and c) connected inseries at 0.5-0.7l/h cm².
 11. A process according to claim 1, whereinthe process is carried out at 20° C.
 12. A process according to claim 1,wherein the process is carried out at 0-10° C.
 13. A process accordingto claim 1, wherein the concentration of organic impurities in thesolution is reduced to less than 5 ppm.
 14. A process according to claim1, wherein the process is conducted at atmospheric pressure.
 15. Aprocess according to claim 1, wherein the concentration of organicimpurities in the solution is reduced to less than 1 ppm.
 16. A processaccording to claim 1, wherein the neutral adsorber resin has amicropore:macropore ratio of up to 1 and a porosity >0.55 g/ml at asurface area/weight unit ratio of less than 600 m²/g.
 17. A processaccording to claim 1, further pre-washing the exchanger and adsorberresins.
 18. A process to claim 4, wherein the ratio between columnheight and column diameter for each column is between 7.5:1-2.5:1.
 19. Aprocess according to claim 1, wherein the equipment used in the processcomprises borosilicate glass, polytetrafluoroethylene, polyvinylidenefluoride or high-pressure polyethylene.
 20. A process according to claim1, wherein the treatment is performed in the order of a), b) and c).